Allowed for adding "null" exports for a timberline poster result; if we want to mark as posted while not actually posting anything.

InABox.Poster.Timberline/TimberlinePostResult.cs

@@ -27,7 +27,7 @@ namespace InABox.Poster.Timberline
         public IEnumerable<KeyValuePair<Type, IEnumerable<IPostableFragment<TPostable>>>> Fragments =>
             fragments.Select(x => new KeyValuePair<Type, IEnumerable<IPostableFragment<TPostable>>>(x.Key, x.Value));
 
-        public void AddSuccess(TPostable post, TExport export)
+        public void AddSuccess(TPostable post, TExport? export)
         {
             post.Post();
             items.Add(new Tuple<TPostable, TExport?>(post, export));